updated image reference for event_display

[hhellbusch]

based on knative/docs#2145

[hhellbusch]

Hi @btison ran into this while working on the RHOS severless foundations lab (section 4.1) -

{"level":"error","ts":1596576851.5154378,"logger":"fallback","caller":"http/transport.go:408","msg":"failed to decode message","error":"unknown encoding","stacktrace":"github.com/knative/eventing-sources/vendor/github.com/cloudevents/sdk-go/pkg/cloudevents/transport/http.(*Transport).ServeHTTP\n\t/go/src/github.com/knative/eventing-sources/vendor/github.com/cloudevents/sdk-go/pkg/cloudevents/transport/http/transport.go:408\nnet/http.(*ServeMux).ServeHTTP\n\t/usr/local/go/src/net/http/server.go:2375\nnet/http.serverHandler.ServeHTTP\n\t/usr/local/go/src/net/http/server.go:2774\nnet/http.(*conn).serve\n\t/usr/local/go/src/net/http/server.go:1878"}

based on https://github.com/google/knative-gcp/blob/master/docs/examples/cloudschedulersource/event-display.yaml - there is a new image to be utilized.

after chaning the image and inspecting the logs from the new pod - things are as expected as per the lab.

cheers,
Henry